The present invention relates to a shadow mask for a Braun tube formed with substantially a rectangular slot and a curved slot for forming a beam, having substantially a rectangular shape, uniformly on a fluorescent surface of a color Braun tube.
FIG. 6 is a general view of a shadow mask for a color Braun tube having a plurality of slots each in substantially a rectangular shape. A conventional shadow mask 61 is composed of a slot forming portion 62 and a skirt forming portion 63. An electron beam passing a slot enters directly linearly into the slot at a central portion S of the shadow mask, but it enters obliquely to the slot as directed to an outer peripheral portion thereof. For this reason, a front side opening and a back side opening of the slot of the conventional shadow mask are adjusted in the opening forming positions thereof.
FIG. 7 is a schematic view showing a positional relationship between the front side opening and the back side openings of the respective slots of the conventional shadow mask. As shown in FIG. 7(i), the central slot of the shadow mask is composed of a front side opening 72 subjected to an etching process with a large area so as not to obstruct the passing of the electron beam and a back side opening 71, which is disposed substantially the central portion of the front side opening 72, as electron beam 73 entering side. On the other hand, the slots on the outer peripheral side of the shadow mask, for example, a slot formed at the point P on the Y-axis of coordinate in FIG. 6, a slot formed at the point R on the X-axis thereof and a slot formed at the point Q on the diagonal coordinate in FIG. 6, are shown in FIGS. 7(ii), (iii) and (iv), respectively, and the front side openings 72 of these slots are arranged so as to be shifted towards the outer peripheral side of the shadow mask 61 with respect to the back side openings 71 so as not to obstruct the passing of the electron beams 73 obliquely entering with respect to the slots.
In the shadow mask of such structure, in order to prevent a thermal deformation (referred to as doming) caused through a collision of the electron beam with the shadow mask, a thin metal plate formed of a material having a small thermal expansion coefficient such as nickel-iron alloy is used as a thin metal plate for the shadow mask. However, because the shadow mask manufactured with such thin metal plate is expensive, a shadow mask formed by making thick a cheap soft steel plate has been used to thereby suppress the thermal expansion of the shadow mask applied to the Braun tube and, hence, to prevent the occurrence of the doming.
The use of the thickened plate for the shadow mask makes large a height of a sectional area of the slot formed through the etching working. For this reason, as the slots formed on the side of the outer periphery as shown in FIGS. 7(ii), (iii) and (iv), with the slots whose front side openings 72 are merely shifted, the obliquely entering electron beam 73 is shut off by the thickened portion of the slot. As a result, the electron beam 73 cannot landed with a predetermined shape on a fluorescent surface of the Braun tube, thus providing a problem.
FIGS. 8 to 10 are schematic views for explaining such problem. FIG. 8(i) is a view showing the shape of the slot formed at the R point on the X-axis of coordinate shown in FIG. 6, in which the front side opening 72 is formed through the etching working with the back side opening 71 being shifted from the front side opening 72. The electron beam 73 passing through the central portion A of the slot can pass, with a desired width W as shown if FIG. 8(ii), a portion between side wall sections 83 and 84 to which thin steps 81 and 82 are formed through the sufficient etching process. On the other hand, the electron beam 73 passing through the upper end portion B in the longitudinal direction of the slot is shut off by a step 86, having a large thickness, formed to a side wall section 88, which is not subjected to the sufficient etching process, as shown by the sectional view of FIG. 8(iii), and hence, this electron beam 73 cannot pass with the desired width W. As mentioned above, the difference of the shapes of the side wall sections, particularly, the thicknesses of the steps are the ventral portion A and the longitudinal upper end portion B resides in the difference in the etching progressing g speeds caused by the positional relationship between the front side opening 72 and the back side opening 71. That is, at the central portion A of the slot, the etching progressing speed is large (high) and this portion is etched with a sufficient speed to thereby form the thin steps 81 and 82. On the other hand, at the upper end portion B, the etching progressing speed is small (low) and this portion is not sufficiently etched, so that the etching progresses from the back side opening 71 having a small opening width and, hence, the steps 85 and 88 having large thickness are formed. As a result, a spot of the electron beam passing the slot and landing on the fluorescent surface provides a curved shape in which upper and lower end portions of a boundary line of the outer peripheral side of the Braun tube because the incident electron beam 73 is shut off by the thickened step 86 formed to the side wall section 88 on the outer peripheral side which is not subjected to the sufficient etching working.
Furthermore, though mentioned hereinlater with reference to FIG. 3, a boundary line 39 (central side of the shadow mask) of an electron beam 31 varies in the electron beam passing position by a back side opening 11 having an enlarged opening area. For this reason, in the case of the slot having substantially a rectangular shape, the electron beam 31 passing the central portion of the slot cannot pass the same position as the boundary line 39 mentioned above, and in the spot landing on the fluorescent surface, both the ends of the longitudinal direction of the boundary line 39 may provide a curved shape towards the central side of the shadow mask.
Accordingly, when the conventional shadow mask 61 is used, both the longitudinal ends of the spot 91 of the electron beam passing the slot and landing on the fluorescent surface provides a shape, as shown in FIG. 9, curved so as to approach the axis of ordinate passing the central portion of the fluorescent surface of the Braun tube. Such deformation of the spot 91 becomes large as the incident angle of the electron beam 73 becomes large, that is, as the electron beam 73 becomes apart from the axis of ordinate mentioned above and is directed in the bilateral direction.
FIG. 10 is a schematic view showing the deformed spot 91 landing on the fluorescent surface of the Braun tube. Such deformation of the electron beam spot 91 may provide a problem of not obtaining sufficient luminance which is obtained in essential by landing the spot on the fluorescent surface with the rectangular shape. Moreover, the shapes of the spots differ from each other at various portions of the fluorescent surface of the Braun tube, so that there is a fear of causing difference in the luminance at different portions and generating an illumination variety of R, G, B colors, thus also providing a problem.
In order to solve the above problems, an object of the present invention is to provide a shadow mask so that a spot of an electron beam landing on a fluorescent surface of a Braun tube takes a desired rectangular shape.
The present invention provides a shadow mask for a Braun tube having a plurality of slots forming beam spots, each having substantially a rectangular shape, uniformly on a fluorescent surface of a color Braun tube, the shadow mask being characterized in that the slots include a slot having substantially a rectangular shape and formed at a portion near an axis of ordinate passing a central portion of the shadow mask and a curved slot formed on an outer peripheral side thereof apart from the axis of ordinate; the rectangular slot is composed of a back side opening formed on an electron beam incident side through an etching process so as to have substantially a rectangular shape, a front side opening formed through the etching process so as to have substantially a rectangular shape having a large area so as to allow the electron beam to pass and side wall sections inclining between the back side opening and the front side opening; the curved slot is composed of a back side opening formed on the electron beam incident side through the etching process so as to be curved such that both longitudinal end portions thereof are apart from the axis of ordinate, a front side opening formed through the etching process so as to have substantially a rectangular shape having a large area so as to allow the electron beam to pass and side wall sections inclining between the back side opening and the front side opening; and the curving of the back side opening of the curved slot becomes large as both the longitudinal end portions are apart from the axis of ordinate.
According to this invention, since the curved slot, which is formed by curving both longitudinal end portions of substantially a rectangular slot so as to be apart from the axis of ordinate passing the central portion of the shadow mask, is formed, the electron beam, which is shielded by the side wall sections of both the longitudinal end portions of the slot having the conventional shape, can pass without being shielded. As a result, both the longitudinal end portions of the spot landing on the fluorescent surface of the Braun tube do not lack. Furthermore, since such curved slot has a long side edge, which is also curved, on the central side of the shadow mask forming the slot, the spot of the electron beam landing on the fluorescent surface of the Braun tube does not vary even in a case where a distance between the end face edges of the back side openings of both the longitudinal end portions of the slot is widened. Still furthermore, the curving of the curved slot becomes large as the slot is apart from the axis of ordinate passing the central portion of the shadow mask, so that it can be possible to follow the variation of the incident angle of the electron beam to the curved slot, and the electron beam spot having substantially the rectangular shape can be formed over the entire area of the fluorescent surface of the Braun tube. Therefore, according to the shadow mask of the present invention, since the spot of substantially the rectangular shape can be uniformly on the fluorescent surface of the Braun tube, the electron beam can be landed on the predetermined position and the lowering of the luminance and the luminescent irregularity can be prevented from causing.
In the present invention of the characters mentioned above, it is preferred that a step is formed to the side wall section of the curved slot in which an etching surface on the front side opening side having a depth gradually reduced towards both the longitudinal end portions from the central portion of the curved slot and an etched surface on the back side opening side having a depth gradually increased are contacted at an intermediate portion in a thickness direction, and the back side opening of the curved slot is provided with end face edges having opposing width which is widened towards both the longitudinal end portions from the central portion of the curved slot.
According to this invention, since the step is formed, to the intermediate portion in the thickness direction of the side wall section of the curved slot, with the etched surface on the front side opening side having a depth gradually reduced towards both the longitudinal end portions from the central portion of the curved slot and the etching surface on the back side opening side having a depth gradually increased, the thickness of the step is thickened towards the both longitudinal end portions of the slot. For this reason, the boundary line, at the outer peripheral side of the shadow mask, of the electron beam passing both the longitudinal end portions of the slot is prevented from passing by the thickened step. However, since the back side opening of the curved slot is formed so that both the longitudinal end portions are curved towards the outer peripheral side of the shadow mask, the electron beam passing both the end portions of the slot can pass the same position of coordinate as that of the boundary line of the electron beam passing the central portion of the slot. As a result, the spot landing on the fluorescent surface has a straight boundary line.
Simultaneously, the back side opening of the curved slot has the end face edges having a widened opposing width towards both the longitudinal end portions from the central portion of the curved slot, so that the end face edge, on the central side of the shadow mask, of the longitudinal end face edges of the back side opening is formed to be parallel to the axis of ordinate mentioned above. As a result, the electron beam entering the curved slot on the central side of the shadow mask constitutes a straight boundary line without being curved, passes the slot and is landed on the fluorescent surface of the Braun tube. As a result, the shape of the spot landing on the fluorescent surface of the Braun tube can be made rectangular without being curved.
Furthermore, it is preferred that the curved slot has an angle of less than 10xc2x0 which is constituted by a curving degree indication line connecting a central point of the curved slot and a central point of an opening width between both the longitudinal end portions and the axis of ordinate passing the central point of the curved slot.
According to this invention, by curving the slot, which is made large as being apart from the axis of ordinate passing the central portion of the shadow mask, at a degree of less than 10xc2x0 with respect to the axis of ordinate passing the central portion of the curved slot, the spot having substantially a rectangular shape can be uniformly formed on the fluorescent surface of the Braun tube.